Balancing of electrical bridge circuits containing nonlinear elements



April 27, 1948. M. M. LEVY 2,440,283

BALANCING OF ELECTRICAL BRIDGE CIRCUITS CONTAINING NON-LINEAR ELEMENTS Filed Nov. 20, 1943 s Sheets-Sheet 1 F/GZ.

F G .4 0 N2 Q N/ VOLTAGE 6 H7.- i lnventor Hal ml! lav]. Y By I Alto ey April 27, 1948. M, LEVY 2,440,283

BALANCING OF ELECTRICAL BRIDGE CIRCUITS CONTAINING NON-LINEAR ELEMENTS Filed NOV. 20, 1945 3 Shets-$heet 2 I I F I00 200 300 JA/ODE V01. 7465 i 1/ VOLTAGE 02 I n venlor pril 27, 1948. M. M. LEVY- 2,440,283

BALANCING OF ELECTRICAL BRIDGE CIRCUITS CONTAINING NON-LINEAR ELEMENTS Filed NOV. 20, 1943 3 Sheets-Sheet 3 I Inventor 0 I60 260 ANODE V0; 73465 300 Maurice Hoi t [fly Altar ey Patented Apr. 27, 1948 BALANCING OF ELECTRICAL BRIDGE CIRCUITS CONTAINING NONLINEAR ELEMENTS Maurice Moise Levy, London, England, assignor,

by mesnc assignments, to International Standard Electric Corporation, New York, N. Y., a

. corporation of Delaware Application November 20, 1943, Serial No. 511,105 In Great Britain April 5, 1943 9 Claims. (Cl. 323-75) l The present invention relates to electrical bridge circuits containing non-linear impedance devices and concerns methods of maintaining the balance or such circuits under varying power supply conditions.

The invention isv or particular application to thermionic valve voltmeters and similar types of apparatus though the same principles are applicable over a wider field, as will be pointed out later on.

In valve voltmeters, the anode impedance of a triode valve forms one arm or a Wheatstone bridge, the other arms of which are usually ordinary resistances. The anode voltage supply is connected to one pair or diagonals of the bridge, and an indicating instrument to the other pair. A voltage to be measurd is applied to the control grid of the valve, and the resistances are com monly adjusted so that when the applied grid voltage is zero, there is no current through the instrument. It is round, however, that the zero adjustment is very sensitive to changes in the voltage of the anode supply, so that the indications are likely to be variable, and the apparatus requires constant re-adjustment. The reason for this is that the anode impedance of the valve is a non-linear resistance for which Ohms law does not hold, and the bridge accordingly goes out or balance when the anode supply voltage changes.

A similar diiliculty arises with any other kind of non-linear impedance device (such as a rectifier or semi-conducting resistance) which forms part of a bridge circuit which is required to remain balanced under some specified condition, when the operating source is subject to variations. It will be understood that the valve voltmeter is only one example of such an arrangement. The object of the invention is to compensate for the effects of changes in the operating source or sources or the like, so that the bridge circuit does not become unbalanced as a result of the proper-' ties of the non-linear device.

According to the invention, there is provided an electrical Wheatstone bridge circuit having a nonlinear impedance device in one of the arms, a

source of operating voltage applied to one pair of diagonal points, and means for biasing the nonlinear device in such manner that the bridge remains balanced while the operating voltage varies over a given range.

According to another aspect, the invention provides an electrical Wheatstone bridge circuit in which one arm includes a nonlinear impedance device and another includes an ordinary resistance, a source of operating voltage being applied to one pair diagonal points Of the bridge, comprising means for biasing the resistance whereby it acquires non-linear properties of such a nature that when the bridge is balanced for a particular value of the operating voltage, it remains balanced when the operating voltage varies over a given range.

According to still another view the invention consists in an electrical circuit arrangement comprising a non-linear impedance device and an ordinary resistance arranged in different arms of a Wheatstone bridge having a source of variable operating voltage applied to one pair of diagonal points, and further comprising biasing means for effectively bringing into substantial coincidence the current-voltage characteristic curves oi the non-linear device and of the resistance,

voltage.

The invention will be described with reference to the accompanying drawings in which- Fig. 1 shows a schematic circuit diagram of a known form of thermionic valve voltmeter;

Fig. 2 shows a generalized schematic circuit diagram of a Wheatstone bridge arrangement containing a non-linear impedance device of any type;

Fig. 3 shows a diagram of curves used to explain the principles of the invention;

Fig. 4 shows a schematic circuit diagram of one embodiment of the invention comprising a valve voltmeter;

Fig. 5 shows characteristic curves used to explain the operation of Fig. a;

Fig. 6 shows further curves used to explain another aspect of the invention;

Figs. 7 and 8 show schematic circuit diagrams of two zero compensating arrangements accordv ing to the invention;

Figs. 9, l0 and 11 show schematic circuit diagrams of valve voltmeters compensated according to the invention; and

Fig. 12 shows typical characteristic curves for a valve which is suitable for use in the embodiment of the invention.

The diiiiculty which arises in bridge circuits containing non-linear impedance devices will be explained with reference to Fig. 1, which shows in its simplest form the circuit of a commonly used thermionic valve voltmeter. A Wheatstone bridge circuit is made up of three resistances R1,

R2, Re, and the anode-cathode impedance of a 3 triode or other type of amplifying valve V. the resistance R being in series with the cathode. A direct current indicating instrument M is con- I nected to terminals l and 2, between the cathode and the junction of resistances R1 and R2, and the anode supply source HT is connected between the anode and the junction of resistances R: and Re which is commonly earthed, as shown. The voltage to be measured is applied between the control grid of the valve V and earth, at terminals 3 and 4, and may be derived from an alternatin voltage or current by rectifying means (not shown). One of the resistances is usually made adjustable so that when the voltage applied to the control grid is zero (or some other standard value) the bridge may be balanced so that the meter M reads zero (or the adjustment may be made so that the meter gives some other standard reading under the standard condition). When a voltage to be measured is applied to the control grid, the anode-cathode impedance of the valve is changed and the condition of balance of the bridge is altered, so that the meter gives a reading which depends on the applied voltage. It is found, however, that if the voltage of the anode supply source changes, the condition of balance of the bridge is also altered and a large zero error often results. This error can of course be removed by re-adjustment, but this is inconvenient, and may even be inpractical if the supply source continually fluctuates.

It is, of course, possible to provide stabilizing arrangements for the H. T. voltage, but such arrangements are often complicated, bulky and expensive, and may be the cause of great inemciency owing to the power consumed by the sta bilizing means. Alternatively, the resistance R1 may be replaced by a dummy duplicate of the valve which, being subject to the same variations of the H. T. voltage, changes in the same manner and thereby maintains the balance of the bridge. This expedient has the objection of cost and complication, and moreover generally results in a less sensitive arrangement, The invention provides compensation of the effect of variations of the supply voltage either by biassing the valve or other non-linear device in such a way that it substantially obeys Ohms law over a small portion of the voltage-current characteristic, which is used as the operating portion, or by biassing the resistance R2 so that its effective characteristic curve is similar to that of the non-linear device. This will be explained presently in detail.

Changes in zero are also produced by variations in the cathode heating source, and by progressive changes in the valve itself. These changes are, however, of negligible importance, so that in practice, only the efiects of the variations of the anode supply source need be considered.

To give an idea of the magnitude of the variations produced, in a particular valve voltmeter it was found that a 10% change in the anode supply voltage produced a change of zero of the order of a quarter of the total range of the indicating instrument.

Fig. 2 shows in general form the bridge circuit of the valve voltmeter of Fig. 1. The block NL indicates any non-linear impedance device to which a voltage or current or other influencing means may be applied for the purpose of changing its impedance. The valve V of Fig, 1 is one example of such a non-linear device. Resistance R3 corresponds to Re. The output terminals I, 2 may be connected to an indicating instrument or to a relay or other utilization device intended produced by the signal on any device connected.

to the terminals I and 2 is interfered with.

It is desirable to give at this stage another example of a non-linear device which would be subject to the same dimculties as a valve. Resistance elements known as thermistors are coming into extensive use at the present time. These elements are composed of semi-conducting materials such as silver sulphide (and others) which have a very large temperature coefilcient of resistance, Such an element is frequently provided with an insulated heating coil intended to be supplied with a control current which may be independent of the current which flows through the resistance element, and the combination is then known as an indirectly heated thermistor. Such a'thermistor will be heated and its resistance changed by the current in the heating coil and also by the current flowing through the resistance element. An indirectly heated thermistor may be used as the device NL in Fig. 2 with the resistance element connected in the bridge arm, the

heating coil being connected to the terminals 3 and t so that a controlling signal current may be supplied thereto. Under some standard condition of the signal current the bridge can be balanced by suitable adjustment of one of the other resistances, but this balance will be destroyed if the voltage of the source S varies, since the change of current through the thermistor will change its resistance.

It is to be noted, also, that since a thermistor is primarily affected by temperature changes it can be used as a thermometer; and when connected in a bridge such as Fig. 2 having a current indicating instrument connected to terminals l and 2 the temperature may be read directly 0n the instrument, and the arrangement will be extremely sensitive. The accuracy of the method depends on the bridge remaining balanced if the applied voltage varies, and this provides another instance of the application of the inven- 171011. It will be seen that in this case the signal or influence applied to the thermistor is a change of temperature rather than an electric current or voltage.

Referring again to Fig. 2, in which the nonlinear device NL is of some type not yet specified, the characteristic curve which gives the relation between the current and voltage for the device may take various forms, and Fig. 3, curve i shows a portion of such a curve.

If the element is operated in the neighbourhood of the point P1 for a very short distance on either side of the point, then the operating portion of the curve is substantially the same as'the tangent QPl at the point P1. The efiective resistance of the element at the point P1 is equal to slope of the tangent, and accordingly the efiective resistance will vary as the operation point moves along the curve, even though its movements remain smell. If, however, the curve can be eflectively shifted so that the tangent at the operating point passes through the origin 0, then the effective resistance will remain substantially constant for small excursions or the operating point.

This may be done either by applying to the elemerit a, blessing voltage equal to Q0 sothat the characteristic curve I moves parallel to the voltage axis to the curve 2, or by applying c. blessing current equal to MiM: so that the characteristic curve moves upwards parallel to the current was to the curve 3. In either case the tangent at the operating point P2 or P: now passes through the origin 0. Which of these two methods should he used depends on the nature of the non-linear element and on the associated circuits.

It will he obvious that the same requirement could he met by applying simultaneous voltage one current bias oi such amount that the charecteristic curve is moved so that it touches the line OPzPs at any other point.

It is to he noted that the characteristic curve oi the device might be shifted in other ways, that is, not necessarily by means of an applied voltage or current. For example, it the nonlinear device is e. thermistor, the shift might be ohteinecl by applying heat to it in some other way, for example by piecing it in an enclosure the temperetiire of which is incle emlently con-- trolled. Am!" appropriate meters of influencing the characteristic oi the nondineer device curve may thus he employed.

While the cherecterlstic of a. non lineer device is in general curved, yet a device may have sub staritielly 2:. straight line characteristic such cs QPl which does not pess through the origin, so that it (ices not obey Chin's low. The term non-linear imperleig'ice device" will cccorclin iv he uses, to include 'olevlce which has it straight line characteristic of? this lrinol, eii'cl which will he suhiect to the ciefects which it is the purpose oi invention to remedy. for the purposes this specification, c non-linear imperlccce cievlce is cleiii'ieri as one which does not they @hmslow.

he understood that ii the hcn lineor le vice is hissed in the manner .rlescrihed so that 6 In Fig. 4, the bridge circuit comprises the resistances R1. R2 and R: which correspond to those of Fig. 2, and the valve V together with its cathode resistance Re, which corresponds to the non-linear device NL. A direct current measuring instrument M is connected to the bridle terminals l, 2 and the high tension supply source corresponds to the source S. An alternating voltage to be measured is applied at the terminals 8 and i to c. diode D (or other rectifying device) through 9. blocking condenser 01 and the rectified voltage across the iced R5 is smoothed by means of the resistance Re and condenser C2 and is applied to the control grid of the valve V which is given an appropriate positive bias by the connection shown between the resistance R5 and an adjustabletep on the resistance Put connected across the E. 'Iflsource a lay-pass condenser Ca being provided.

In Fig. 5 are sl'iowri some curves relating the anode potential of the valve V to the corner through the resistance Re when the grid is mete trained at earth potential. Curves are given f r several aililerent values of Be as indicated. The valve V to which the curves of Fig. 5 apply is a trlocie having the characteristic curves given tcne'eiit sit the operatingpoirit of the char cot the efiectlve resistance will he suhstentially in ciepencient of the voltage of the source the hririge *1? remain balanced.

li ifcen the hi go is balanced, iii/Pea must he cocci to Rte/1 33, where Re is the eiifcctive once of the nondineer device. Unless R2=Rs the characteristic curves of its cool R1 will have slopes, when the bridge h ericecl. It will however be convenient to consi er the resistances one; R1 as though R2'- and reccording-"iv the slope of the characteristic of resist c it! (or R0) when connected with that of 3th) will he called the correcteci slope that is, corrected for the hriolge ratio, so that the slope of one resistance will he seicl to he ii cocci to the corrected slope of smother. Lice the characteristic will he referred to es the corrected characteristic.

in. Fig. is shown the circuit of e. valve volt meter incorporating arrangements according to the invention of the lclnol just described. It is adapted for measuring alternating voltages end includes a rectifying arrangement for deriving the voltage which is applied to the control grid of the valve.

etic curve passes through the origin G. then Fig". 12, which shows the relation between the anode current and. voltage for various values of the grid voltage with respect to the cathode. will he noted that the curves of Fig. 5 are suhstentielly straight lines except very near the origin, and, ii? these straight lines were producer? backwards they would cut the voltage er s points the neeetive side of the current '21s. The curve for Fe=3ilcil ohms hasbeeii producer? backwards lay dotted. line to 5?? and meets the voltage axis at the point It can easily he shown that when the grid potentia chem-reel negatively the curves are all suhstem tiellgr translated; to the right parallel to the volt age axis. has by appropriate choice of gricl voltage, the coco ohm line, for example, be

e to pass through the origin 6. In peritier case 9, potential of -03 volt on t wou d he required to do this. use 01 LJ..LS griol peter? the reflective resistance of the valve together with. the cathode resistance will he substantially ir ep-enrieht of the eho-cie voltage, one"; therefore independent the voltage o the tension source. Any of the curves 0 El moi," he similarly shifted by an 2. choice or gricl Referring to 3, when no alternating voltis applied to the slicclc D, there will be e smell the resistance R5 which will bias the control oi the valve v 3.1 etivelv. It prohehly be four this more ti rec to shift the chart. ties .eci manner. For this i resistance is. provided, who counterecine iiositive bias in. tlie in setting the circuit, the resistance P1 is first eclliistecl with some reacting is prociuceo "he a. the callu on b meter which remains constant while the iii 11 tension voltage is chengecl. This adjustment makes the corrected slope of the characteristic of R1 equal to the slope of the relevant characteristic curve oi. Fig. 5. The proper high tension voltage is then applied. and. the grid bias is adjusted by means of the tapping on the resistehce R4 until this meter reading is reduced to zero. This shifts the characteristic of Fig. v5 until it substantially coincides with the cor-- rected characteristic of the resistance R1. It

will then be found that the reading or the indicating meter M corresponding to an alternating voltage applied at terminals 3 and l is practically unaffected by changes in the high tension voltage.

It will be evident that any other suitable known arrangement for adjusting the grid bias may be used instead of the one shown in Fig. 4.

This circuit has been found to give good results in practice, but its scope is limited. The sensitivity desired is obtained by suitably selecting Re, and the high tension voltage being given, the plate current of the valve is then determined at the value indicated by the appropriate curve of Fig. 5 as shifted, which value may be too small. For this reason an alternative method or proceeding according to the invention is preferred. The arrangement is then substantially the same as Fig. 1 except that the resistance R2 is made to behave like a non-linear device with a characteristic similar to that of the valve,

so that the bridge remains balanced when the I high tension voltage is changed.

Referring again to Fig. 1, in order that the meter M shall show no deflection as the HT voltage varies, the potential drop across the resistances Re and R2 must be the same at all times. The curves of Fig. 5 show the relation between the anode voltage and the anode current I for the valve V andresistance R0, as already explained. The potential drop across the resistance Rc will be I30, and if there is to be no current in the meter M, then the current in R3 must be I.Rc/R2 however the HT voltage may vary. If the curve for Rc=3000 ohms be taken as an example, this means that the relation between the current and the HT voltage for the resistance R: must be represented by a line which passes through the point P but which has a slope less than that of the 3000 ohm line in the ratio Rz/Rc; in other words the slope must be cor= rected for the bridge ratio.

In Fig. 6, line i shows as an example the working portion of the 3000 ohm characteristic redrawn to different scales, and produced backwards to meet the voltage axis in the point P as before. Curve 2 shows the characteristic required for the resistance R2 passing through the point P and having a smaller slope, it being assumed, for example, that R2 is greater than Re. Curve 3 shows the normal characteristic for the resistance R2 which passes through the origin 0, it being remembered that the voltage scale refers to the voltage applied to the diagonal points of the bridge and not to the voltage at the terminals of R2. Since curve 2 is the desired characteristic for the resistance R2, it is necessary by some means efiectively to shift the curve 3 upwards into coincidence with curve 2.

It will be remembered in connection with Fig. 4, that the diode D applies a negative bias to the control grid of the valve V, which has the effect of shifting all the curves of Fig. 5 to the right so that they intersect the voltage axis on the positive side of the current axis. In cases like this, the characteristic desired for R2 would be similar to the line 4 of Fig. 6, requiring the characteristic 3 to be shifted downwards instead of upwards.

It will be seen that the current corresponding to any point on line 2 is the sum of two currents, the first of which is proportional to the applied voltage and is given by the line 3, and the second of which is constant and given by the length OQr. Similarly in the case of line =4, the ourfor Fig. 7, and

rent is the difierence between the current represented by the line 3 and a constant current OQz.

Figs. 7 and 8 show arrangements according to the invention for shifting the characteristic line 3 upwards and downwards respectively. This result is accomplished particularly by impressing a substantially fixed current differential in the linear impedance. Referring to Fig. 7, the resistances R1 and R2 are shown connected across the high tension supply as in Fig. 1. A neon tube N (or other gas filled tube or like device) is connected in series with a resistance R1 across the high tension supply, the tube being connected to the negative terminal, and the junction point is connected to-terminal 2 through the resistance Ra. The neon tube acts so that a constant potential is applied to the resistance Re which is independent of the variations of the high tension voltage.

Fig. 8 shows. the arrangement for shifting the characteristic line downwards. It difiers from Fig. 7 in that the resistance R7 is omitted, and that the neon lamp is connected in place of it to the positive terminal of the HT supply. In Figs. 7 and 8, let i1, 2'2 and is be the current flowing in the resistances R1, Ba and R3 respectively as shown. Let V1 be the high tension voltage, and v the constant voltage maintained by the neon tube N. Then it is easily shown that for Fig. 7

Since the term in v is constant, and of opposite signs in these two expressions, it will be evident that the relation between V1 and 1!: corresponds to the line 2 of Fig. 6 in the case of Fig. '7 and to the line Q in the case of Fig. 8. Preferably R3 should be chosen to be small compared with R1 and Rs in which case the two expressions reduce approximately to for Fig. 8. Thus in the case of Fig. 7, R1 may be lid adjusted to obtain the desired slope for the line 2, and R8 may then be adjusted independently so that the intercept 0Q has the desired value. Unfortunately in the case of Fig. 8, the adjustments of R1 and Re can no longer be independent and the necessary setting is more trouble-= some to obtain. For this reason, as will be explained later, it is preferable to employ the arrangement of Fig. 7 together with a grid blessing potentiometer similar to that shown in Fig. 4.

It may be pointed out that in Fig. 8 it is possible to omit Ra altogether, so that the neon tube N is connected direct to terminal 2. The curve 5 of Fig. 6 is in this case produced, and this curve cuts the voltage axis at the fixed point Vn where 0V is the voltage required to maintain the discharge of the tube. In other words for a given neon tube, the curve 5 is fixed, and it would then be necessary to apply appropriate bias to the control grid of the valve so that the curve d is moved into coincidence with curve 5.

It will be understood that if necessary, two or more neon or other gas filled tubes may be used in series if one tube does not provide a large enough constant voltage to produce the current is. It will be appreciated that in the arrangements of Figs. 7 and 8 the resistance R2 has been given 9 the properties of a non-linear impedance device as defined in this specification.

Fig. 9 shows a complete circuit similar-to Fig. 1 provided with zero balancing arrangements according to Fig. 7. The control grid of the valve V is connected to earth through a grid resistance R so that in the absence of any voltage applied to the terminals 3 and 4 it will be substantially at earth potential, and the curves of Fig. 5 therefore apply. The various other elements are given the same designations as in Figs. 1 and '7. In making the zero settings, the resistance Re is first set at some value in order to get a meter deflection, and R1 is then adjusted so that when no voltage is applied at terminals 3 and 4 the meter M. gives some reading which does not vary when the voltage of the HT supply is changed. This gets the slope of the curve 2 (Fig. 6) correct. The resistance R8 is then adjusted until the meter reads zero. This shifts the curve to the desired position.

In a particular case of this circuit the following components were used:

Valve V.-=Typical characteristic curves are shown in Fig. 12.

Meier lib-Range (ll-200 microamperes- Neon tuba-stabilising voltage about iii volts.

With this arrangement in proper adjustment, a full scale deflection on the meter M (2% micro amperes) was produced when a potential oi -il.25 volt was applied to the control grid oi the valve V, and the variation of zero was $0.3 micro ampere for a variation of :50 volts of the it. T. voltage, the average value being use volts.

Fig. shows the one current of Fig. 8 applied to the circuit when provided with a. rectifying diode D as in Fig. i. in a specific case the com ponents used were the same as in Fig. 9 en R1=93,09-ii chins (variable) and Re=l8miflll ohms (variable). In addition, Rs=Rc=l megohm and 02:0.01 microfarad. The diode produced a potential of about -1 volt on the grid in the ab sence of signals applied to terminals 5. This circuit was adjusted according to the same principles as Fig. 9, but as already explained, adjustments of R1 and Rs are not indepencent. For this reason the arrangement of Fig. was preferred. This will be seen to be provideclwith an adjustable counteracting bias for the con .Jol grid obtained from a potentiometer Re, Rio shunted across the neon tube i i, and the zero balancing arrangement of Fig. "Z is used.

In particular example of the circuit of the valve V, meter M, neon tube N, and the diode D were as before specified, and the various other elements were as follows:

R1=23IL000 ohms (variable) 532:2,009 ohms Re=l0fililil ohms Rv=70,illl0 ohms Ra=27,000'0hll15 (variable) 3%:1 megohm R1o=l00,000 ohms :10 megohms Rs=2 megohms C2:0.01 microfarad.

With this arrangement a full scale deflection of the meter M (200 microamperes) was obtained iii by the application ofan alternating potential of 0.6 volt to the terminals 3 and l. and the zero stability obtained was about :1 microampere when the high tension voltage was varied :50 volts from 250 volts. The same voltage variation was found to produce only a 2% variation of sensitivity at full scale deflection. The stability oi. the calibration of the arrangement is principally due to the high negative feedback produced by the resistances Ru and Re which have been given larger values than previously for this reason.

Although in the embodiments described triode valves have been shown, it is clear that valves having any number of additional electrodes or grids suitably polarized in known manner, could be used.

It will be understood from the explanations which have been given that the balance of the bridge containing the non-linear device is maintained during variations of the applied voltage either by bias-sing the non-linear device so that at least overits working range it behaves substantially like an ordinary resistance which obeys Ohms law, or by blessing one oi the bridge resistances so that it hehaves like a non-linear device having the characteristics necessary to maintain the balance. In this statement the term blessing means the application of a voltage or current or other appropniate influence in such way that the characteristic curve of the device is eilectively shifted with respect to the co-ordihate axes, without using substantially altered in shape. Although the invention has been described in terms of embodiments employing a valve as the non-linear device it will he evident that the same principles are applicable to other kinds of non-linear impedance devices, though the details of the methods of applying" the principles will very lihelv he different. Having given the nature oi the newlines-r device, it will he evident to those skilled the art from the explanations which have s given how the principles of the invention slacc. be to What claimed is:

l. A bridge clrciut which remains balanced when the openttlng voltage varies comprising means including a non linear impedance constitilting one bridge arm a linear impedance in series therewith constituting" anotl'ler e.

I second linear impedance constituting the balanc lug arm therefor in the bridge, and means for imparting to the second linear impedance 9, suhstantially constant current voltage characteristic irrespective of substantial variations in the oper ating voltage from its normal. voltage and sub-- stantially equivalent to the characteristics oi said non linear impedance and. in. accordance with the bridge scale.

:2. A bridge circuit which remains lzzalanced when the operating voltage varies comprising cans including a'ncndtnear impedance consti tuting one bridge arm and a linear impedance in series therewtth. constituting; another arm, a

means including a non-linear impedance constituting .on bridge arm and a linear impedance in series therewith constituting another arm, a second linear impedance constituting the balancing arm therefor in the bridge, and a shunt circuit for impressing a substantially constant current differential on said second linear impedance irrespective or substantial variations in the operating voltage from its normal voltage.

4. A bridge circuit which remains balanced when the operating voltage varies comprising -means including a space discharge tube having connection from the resistance means to the control grid of the space discharge tube.

5. An electrical Wheatstone bridge circuit, in combination, in which one arm includes a nonlinear impedance device and another includes an ordinary resistance, asource of operating voltage applied to one pair of diagonal points of the bridge,'means connected to the other diagonal points and responsive to a difierence in voltage thereacross, and means continuously blessing the resistance with a constant current voltage characteristic corresponding with the non-linear characteristic of the non-linear impedance whereby when the bridge is balanced for a particular value of the operating voltage, it remains balanced when the operating voltage varies over a given range.

6. An electrical Wheatstone bridge circuit, in combination, in which om arm includes a nonlinear impedance device and another includes an ordinary resistance, a source of operating voltage applied to one pair of diagonal points of the bridge, means connected to' the-other pair of diagonal points and responsive to a difference in voltage thereacross, means for continuously blessing the resistance with a. constant current bias of such a magnitude that the current voltage characteristic curve of the resistance is efiectively shifted into substantial coincidence with the operating portion of the characteristic curve of the non-linear device after correction for the bridge ratio whereby when the bridge is balanced for a particular value of the operating voltage, it remains balanced when the operating voltage varies over a given range.

7. A electrical Wheatstone bridge circuit, in combination, in which one arm includes a nonlinear impedance device and another includes an ordinary resistance, a source of operating voltage applied to one pair of diagonal points of the bridge, means connected with the other diagonal points and responsive to a difference in voltage thereacross, means constantly biassingthe resistance with a constant current bias of such a magnitude that the current voltage characteristic curve of the resistance is effectively shifted into substantial coincidence with the operating portion of the characteristic curve. of the nonlinear device aiter correction for the bridge ratio, including a gas-filled discharge tube connected to the operating voltage source and maintaining a constant voltage which is independent of the variation of the operating voltage whereby when the bridge is balanced for a particular value of the operating voltage, it remains balanced when the operating voltage varies over a given range.

8. An electrical Wheatstone bridge circuit, in combination, in which one arm includes a. nonlinear impedance device and another includes an ordinary resistance, a source of operating voltage applied to one pair of diagonal points of the bridge, means connected to the other diagonal points and responsive to the difference in voltage thereacross, means for blessing the resistance including a gas filled discharge tube, and a resistance separate from the bridge circuit connected in series therewith, the gas filled tube and separate resistance being connected across the operating voltage source, the resistance being connected to the positive side of said source, and a second resistance connecting the juncture of said tube and said separate resistance to the end of said ordinary resistance which is connected to one of the other diagonal points of said bridge whereby the ordinary resistance acquires nonlinear properties of such a nature that when the bridge is balanced for a particular value of the operating voltage, it remains balanced when the operating voltage varies over a given range.

9. An electrical Wheatstone bridge circuit in combination in which one arm includes a nonlinear impedance device and another includes an ordinary resistance, a source of operating voltage applied to one pair of diagonal points of the bridge, means connected to the other d agonal points and responsive to a difference in voltage thereacross, and means for blessing the resistance including a gas-filled discharge tube, and a resistance separate from the bridge connected in series therewith, the gas filled tube and separate resistance being connected between the positive side of the operating voltage source and the end of said ordinary resistance connected to one of said diagonal points of said bridge.

MAURICE MOISE LEVY.

nnrnannors errant The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,966,047 Byall July 10, 1934 1,969,518 Moles Aug. 7, 1934 2,008,855 Drobish July 23, 1935 2,147,446 Koch Feb. 14, 1939 2,147,729 Wurmser Feb. 21, 1939 2,190,743 Vance Feb. 20, 1940 2,289,951 Yeutter July 14, 1942 2,305,307 Wellenstein et al. Dec. 15, 1942 2,311,807 Anderson Feb. 23, 1943 

